Tank cap and tank cap apparatus

ABSTRACT

A fuel cap ( 10 ) closes the fuel supply inlet (FNb) of a filler neck (FN) at a narrow operating angle (about 90°), improving the sealing properties of a gasket (GS). The fuel cap ( 10 ) has a cap engagement element ( 20   a ), which is brought into engagement with the opening engagement element (FNc) of the filler neck (FN), and seals the gap around the filler neck (FN) by means of the gasket (GS). The opening engagement element (FNc) is formed in an inclined state at a predetermined angle relative to the direction orthogonal to the axial direction for closing the cap. The cap engagement element ( 20   a ) has a guide surface ( 20   b ). This surface is aligned and brought into engagement with the opening engagement element (FNc) by the rotation of the fuel cap ( 10 ) in the closing direction while this element is inserted into the filler neck (FN). The guide surface ( 20   b ) has a first inclined portion ( 20   c ) with a considerable inclination α2, and a second inclined portion ( 20   d ) whose inclination α1 is less than α2.

BACKGROUND OF THE INVENTION CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority from JapaneseApplications No. 2000-169913 filed Jun. 7, 2000 and No. 2001-98077 filedMar. 30, 2001, the contents of which are incorporated herein byreference.

[0002] 1. Field of the Invention

[0003] The present invention relates to a tank cap to close a tankopening member and a tank cap apparatus with the tank cap attachedthereto.

[0004] 2. Description of Related Art

[0005] In conventional practice, tank caps are configured such that afuel supply port is closed by rotating a fuel cap fitted with a gasketseveral times relative to a filler neck connected to a fuel tank. Sincerotating the fuel cap a plurality of times sometimes fails to result ina tight fit, closing the fuel supply port of a filler neck with a fuelcap merely by turning the cap through a predetermined angle (forexample, about 90°) has been proposed as a way of overcoming thisshortcoming.

[0006]FIG. 24 is a diagram depicting the state existing before a fuelcap is mounted over a filler neck. The fuel cap 100 comprises a casingmain body 102 for opening and closing the fuel supply inlet FNb of afiller neck FN, a cover 104 mounted on the casing main body 102, and agasket GS mounted in the upper portion of the casing main body 102. Aratchet mechanism (not shown) is interposed between the casing main body102 and the cover 104 to ensure that the cover 104 idles relative to thecasing main body 102 when excessive torque is applied between the cover104 and the casing main body 102.

[0007] A casing engagement element 102 a is also formed in the lowerportion of the outer circumference of the casing main body 102. Anopening engagement element FNc is further formed in the innercircumferential portion of the filler neck FN. Part of the area aroundthe inside of the opening engagement element FNc is provided with a neckinsertion notch FNd for allowing the casing engagement element 102 a ofthe fuel cap 100 to be inserted in the axial direction.

[0008]FIG. 25 is a diagram depicting the manner in which the fuel cap100 is mounted over the filler neck FN. The opening engagement elementFNc is tapered by a prescribed inclination α in the axial direction, andthe guide surface 102 b of the casing engagement element 102 a istapered to match this angle.

[0009] The manner in which the inlet FNb of FN is closed with the fuelcap 100 will now be described. The casing engagement element 102 a ispositioned in the neck insertion slot FNd, and the fuel cap 100 isturned through a predetermined angle (about 90°) while the fuel cap 100is inserted into the filler neck FN. By this, the fuel cap 100 ismounted over the filler neck FN as a result of the fact that the casingengagement element 102 a is caused to align with and engage the openingengagement element FNc. At the same time, the gasket GS is compressedbetween the casing main body 102 and the filler neck FN, forming a seal.If rotated further, the cover 104 will idle due to the presence of theratchet mechanism, signaling that the limit has been reached andindicating that the fuel cap 100 fits tightly over the inlet FNb.

[0010] Reducing the inclination α of the opening engagement element FNcduring the closure of the fuel cap 100 will increase the operating angleof the fuel cap 100, not only making the cap more difficult to operatebut also allowing the fuel cap 100 to occasionally rotate through anangle greater than 180° and come off. Conversely, increasing theinclination α will increase the tightening rate of the gasket GS (thatis, the extent to which the gasket GS is compressed) and will enhancethe rubber recoil of the gasket GS. For this reason, the rotationaltorque will increase and the cover will be idled by the ratchetmechanism before the gasket GS is adequately compressed, making itimpossible to form a tight seal.

[0011] This approach is thus disadvantageous in the sense that it isdifficult to cause the gasket GS to form a tight seal by rotating thefuel cap 100 through a narrow operating angle (about 90°). In addition,considerable load is applied between the opening engagement element FNcand the corner 102 c of the casing engagement element 102 a when thecorner 102 c is pressed against the opening engagement element FNc. Agreater rotational torque is therefore needed to close the fuel cap 100,sometimes making it impossible to close the cover properly and form atight seal.

SUMMARY OF THE INVENTION

[0012] One aspect of the present invention provides a tank cap apparatusin which a seal member can form a better seal with a small operatingangle.

[0013] In accordance with one embodiment of the present invention, atank cap apparatus comprises a tank opening member having an openingengagement element and a cap constructed and arranged to close the tankopening member through rotation. The cap includes a cap engagementelement for engaging the opening engagement element and a gasketinterposed between the cap and a sealing surface of the tank openingmember and constructed and arranged to seal the gap therebetween. Theopening engagement element is inclined at a predetermined angle relativeto a direction orthogonal to an axial direction for closing the cap andthe cap engagement element has a guide surface for engaging the openingengagement element by a rotation of the cap in a closing direction whilethe cap is inserted into the tank opening member. An angle of the guidesurface decreases with a rotation of the cap in the closing direction.

[0014] In the tank cap apparatus according to the present invention,inserting the cap into the tank opening member and rotating the cap inthe closing direction will seal the gap between the cap and the tankopening member. This is because a cap engagement element will engage andfit over an opening engagement element, and the seal member interposedbetween the cap and the tank opening member will be compressed.

[0015] During cap closure, the guide surface of the cap engagementelement aligns with and engages the opening engagement element. Theguide surface is configured such that the angle thereof diminishes asthe cap is rotated in the closing direction during this process.Specifically, the configuration is selected such that the inclination isgreater in the initial stage. Despite the fact that the inclination ofthe guide surface is thus increased in the initial stage and thetightening rate is kept at a higher level, the torque is prevented frombecoming excessively high and the seal member can be provided with anadequate tightening margin because the seal member produces only a weakreaction force during the initial compression stage of the seal member.When the seal member is compressed past the initial stage, the guidesurface acquires a lesser angle and becomes capable of consistentlyproviding a better seal in a state in which the tightening rate of theseal member is reduced.

[0016] Accordingly, aligning the steeply inclined guide surface with theopening engagement element during the initial compression stage of theseal member makes it possible to provide the seal member with anadequate tightening margin (and hence to obtain a tight seal) withoutcreating a strong reaction force even in the case of a narrow rotationangle.

[0017] As a preferred embodiment of such a guide surface, a simplestructure can be formed by providing a first inclined portion configuredat an inclination whose angle with respect to the direction orthogonalto the axial direction is greater than the angle of the openingengagement element, and providing a second inclined portion formed as acontinuation of the first inclined portion and inclined at an anglesubstantially equal to the aforementioned predetermined angle. The capengagement element can be easily formed with high precision by beinginjection-molded monolithically with the casing main body despite beingconfigured with a slightly varying incline in order to obtain the firstand second inclined portions. In addition to the configuration in whichthe second inclined portion is formed as a surface that continues pastthe first inclined portion, it is also possible to adopt a configurationin which a third inclined portion is provided at an inclination that isintermediate between those of the first and second inclined portions.

[0018] According to another embodiment of the guide surface, it ispossible to provide a structure in which the angle of inclinationrelative to the direction orthogonal to the axial direction is greaterthan the predetermined angle of the opening engagement element. Inaddition to being fashioned as a planar surface of constant inclination,the guide surface may also be fashioned as a curved surface orconfigured in accordance with a variety of other possible embodiments inwhich a continuously varying inclination is adopted.

[0019] Preferred embodiments of the seal member include C-rings,O-rings, and various other types of seal members characterized byexerting weak reaction force when compressed in the initial stage.

[0020] In accordance with another embodiment of the preset invention, atank cap for closing a tank opening member provided with an openingengagement element formed in an inclined state at a predetermined anglecomprises a casing main body including a flange formed in an upperportion of the casing main body, a gasket holder disposed underneath theflange, and a cap engagement element formed underneath the gasketholder. The gasket is held by the gasket holder and seals a gap around asealing surface of the tank opening member. The cap also includes acover rotatably mounted over the flange. The cap engagement element hasa guide surface for engaging the opening engagement element by arotation of the cap in a closing direction while the cap is insertedinto the tank opening member. An angle of the guide surface decreaseswith a rotation of the cap in the closing direction.

[0021] In the tank cap apparatus according to the present invention,inserting the cap into the tank opening member and rotating the capthrough an angle 180° or less in the closing direction will cause a capengagement element to engage and fit into an opening engagement elementand to seal the gap between the cap and the tank opening member bycompressing the gasket interposed between the cap and the sealingsurface of the tank opening member.

[0022] As the cap engagement element projects from the outercircumference of the cap, a gap is left between the outercircumferential surface of the cap and the inner wall of the tankopening member in the area not covered by the cap engagement element,but this gap is filled with a tilt-preventing rib. An outsidecap-tilting force pushes the tilt-preventing rib against the inner wallof the tank opening member, allowing the cap to tilt only slightly.Accordingly, the gasket retains its sealing properties withoutundergoing excessive plastic deformation.

[0023] It is also possible to adopt a structure in which thetilt-preventing rib is disposed above the area not covered by the capengagement element. Furthermore, the arrangement in which thetilt-preventing rib is formed around the entire cap periphery can beabandoned in favor of an arrangement in which the rib is provided individed form at locations that are best suited for the prevention of captilting. In addition, extending the tilt-preventing rib to a heightsubstantially equal to or greater than the radial height of the capengagement element causes the rib to come into contact with the innerwall of the tank opening member and to prevent the cap from tilting andthe cap engagement element from coming into contact with the inner wallof the tank opening member when a cap-tilting force is applied.

[0024] According to another embodiment of the invention, there isprovided a tank cap for closing a tank opening member by rotating thecap through an angle of 180° or less to bring the cap into engagementwith an opening engagement element formed on the inner wall of the tankopening member, wherein this cap comprises a casing main body comprisinga flange formed in the upper portion thereof, a gasket holder disposedunderneath the flange, and a cap engagement element fashioned as aprojection underneath the gasket holder. The cap also comprises a gasketheld by the gasket holder and designed to seal the gap around thesealing surface of the tank opening member and a cover rotatably mountedover the flange. The casing main body further comprises atilt-preventing rib formed as a projection on the outer circumferentialportion of the casing main body above the cap engagement element butbelow the gasket, and designed to come into contact with the inner wallof the tank opening member and to prevent the casing main body fromtilting upon application of a force that tends to tilt the casing mainbody.

[0025] In addition to the means whereby the cap and the openingengagement elements are engaged merely by being rotated through an angleof 180° or less, a torque mechanism for achieving an engagement througha prescribed rotational torque can be provided as a means whereby thecap engagement element is brought into engagement with the openingengagement element by rotating the tank cap through an angle of 180° orless.

[0026] These and other objects, features, aspects, and advantages of thepresent invention will become more apparent from the following detaileddescription of the preferred embodiments with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a side view in partial section illustrating a tank capin a first embodiment according to the present invention;

[0028]FIG. 2 is an exploded view that shows a casing engagement elementof the casing main body that engages with a filler neck;

[0029]FIG. 3 is and enlarged partial side view that shows a casingengagement element of the casing main body that engages with a fillerneck.

[0030]FIG. 4 is an exploded perspective view illustrating a cover and atorque mechanism disposed in the upper portion of the casing main body;

[0031]FIG. 5 is a sectional view illustrating the torque mechanism,taken on the line V-V in FIG. 1;

[0032]FIG. 6 is a schematic diagram that shows an initial state in aseries of operations of the torque mechanism in the first embodiment;

[0033]FIG. 7 is a schematic diagram that shows a state subsequent to thestate of FIG. 6 in the series of operations of the torque mechanism ofthe first embodiment;

[0034]FIG. 8 is a schematic diagram that shows a state subsequent to thestate of FIG. 7 in the series of operations of the torque mechanism ofthe first embodiment;

[0035]FIG. 9 is a schematic diagram that shows a state subsequent to thestate of FIG. 8 in the series of operations of the torque mechanism ofthe first embodiment;

[0036]FIG. 10 is a schematic diagram that shows a state subsequent tothe state of FIG. 9 in the series of operations of the torque mechanismof the first embodiment;

[0037]FIG. 11 is a schematic diagram that shows a state subsequent tothe state of FIG. 10 in the series of operations of the torque mechanismof the first embodiment;

[0038]FIG. 12 is a schematic diagram that shows a state subsequent tothe state of FIG. 11 in the series of operations of the torque mechanismof the first embodiment;

[0039]FIG. 13 is a schematic diagram that shows a state subsequent tothe state of FIG. 12 in the series of operations of the torque mechanismof the first embodiment;

[0040]FIGS. 14A, 14B, 14C, and 14D are partial enlarged side viewdiagrams depicting a series of operations for bringing the capengagement element into engagement with the opening engagement element;

[0041]FIG. 15 is a graph showing the relation between gasket reactionand the amount of compression (tightening margin) during gasketflattening;

[0042]FIG. 16 is a partial enlarged side view that shows the capengagement element pertaining to another embodiment;

[0043]FIG. 17 is an exploded side view in partial section that shows thestate existing before a fuel cap pertaining to another embodiment of thepresent invention is mounted over a filler neck;

[0044]FIG. 18 is a partial enlarged cross-sectional view of the fuelcap;

[0045]FIG. 19 is an enlarged schematic side view that shows the ribsformed on the outer circumferential surface of the casing main body;

[0046]FIGS. 20A, 20B, 20C, and 20D are schematic side views that showthe operations involved in the closing of the fuel cap;

[0047]FIGS. 21A and 21B are schematic side views that show theoperations involved in the opening of the fuel cap;

[0048]FIG. 22 is a partial side view in cross section that shows thestate in which an outside force is applied to the fuel cap from below;

[0049]FIG. 23 is a top schematic diagram that shows the effectdemonstrated by the outside force shown in FIG. 22 when this force isapplied to the fuel cap;

[0050]FIG. 24 is an exploded view of the state existing before aconventional fuel cap is mounted over a filler neck; and

[0051]FIG. 25 is an enlarged partial side view showing the manner inwhich a conventional fuel cap is mounted over a filler neck.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0052]FIG. 1 is a partial sectional view illustrating a fuel cap 10 in afirst embodiment according to the present invention. The fuel cap 10 isattached to a filler neck FN having a fuel supply inlet FNb (tankopening member), through which a supply of fuel is fed to a fuel tank(not shown). The fuel cap 10 includes a casing main body 20 preferablymade of a synthetic resin material such as polyacetal, a cover 40mounted on an upper portion of the casing main body 20, which ispreferably made of a synthetic resin material, such as nylon, and workslike a handle, an inner cover 50 that closes an upper opening of thecasing main body 20 to define a valve chest 23, a positive pressurevalve 60, and a negative pressure valve 70 that are accommodated in thevalve chest 23 and work as pressure control valves, a torque mechanism80, and a gasket GS set on an upper circumference of the casing mainbody 20 to seal the casing main body 20 against the filler neck FN.

[0053] The elements of the fuel cap 10 in this embodiment are describedin detail below. The casing main body 20 has a substantially cylindricalouter tubular body 21 with a casing engagement element 20 a, whichengages with an another engagement element disposed at innercircumference of the filler neck FN, and a valve chest-forming body 22that is disposed inside the outer tubular body 21. The valvechest-forming body 22 defines the valve chest 23, in which the positivepressure valve 60 and the negative pressure valve 70 are accommodated.

[0054] The gasket GS is mounted on the lower surface of a flange 33formed in the upper portion of the casing main body 20. The gasket GS isinterposed between a seal support 21 a of the flange 33 and the fuelsupply inlet FNb of the filler neck FN. When the fuel cap 10 is fit intothe fuel supply inlet FNb, the gasket GS is pressed against the sealsupport 21 a to exert the sealing effect. The casing engagement element20 a is formed in the lower portion on the outer circumference of theouter tubular body 21. FIG. 2 shows the casing engagement element 20 aof the casing main body 20 that engages with the filler neck FN. Anopening engagement element FNc is formed on the inner circumference ofthe filler neck FN. A neck insertion notch FNd is formed in a part ofthe inner circumferential portion of the opening engagement element FNcin order to receive the casing engagement element 20 a of the fuel cap10, which is axially inserted into the fuel supply inlet FNb of thefiller neck FN.

[0055]FIG. 3 is a diagram depicting the relation between the openingengagement element FNc and the cap engagement element 20 a extendingfrom the outer circumferential surface of the casing main body 20. Theopening engagement element FNc is formed at a first inclined angle α1relative to the direction orthogonal to the axial direction in which thefuel cap 10 is advanced and retracted. The cap engagement element 20 ais provided with a guide surface 20 b. The guide surface 20 b comprisesa first inclined portion 20 c, which is formed at a second angle α2greater than the first angle α1, and a second inclined portion 20 d,which is formed as a continuation of the first inclined portion 20 c andis inclined at an angle equal to the first angle α1. Thus, the capengagement element 20 a has a generally tapered or trapezoidal shape.

[0056] Turning the fuel cap 10 by a predetermined angle (about 90°)while the cap engagement element 20 a is disposed in the neck insertionnotch FNd and the fuel cap 10 is inserted into the filler neck FN causesthe cap engagement element 20 a to move in the direction of arrow D1 andto engage the lower surface of the opening engagement element FNc,whereby the fuel cap 10 is mounted over the filler neck FN. The reasonthat the guide surface 20 b comprises the first inclined portion 20 cand the second inclined portion 20 d will be described below withreference to the operation involving the opening and closing of the fuelcap 10.

[0057]FIG. 4 is an exploded perspective view illustrating the cover 40and the torque mechanism 80 disposed in the upper portion of the casingmain body 20. FIG. 6 is a schematic sectional view illustrating thetorque mechanism 80, taken on the line V-V in FIG. 1. The torquemechanism 80 gives the user a feeling of attachment by an audible clickwhen the rotational torque of a predetermined level or more is appliedto the cover 40 when closing the fuel supply inlet FNb with the fuel cap10. The user can thus confirm that the fuel cap 10 is attached to thefiller neck FN with the rotational torque of the predetermined level ormore.

[0058] Referring back to FIG. 1, the cover 40 is detachably attached tothe flange 33 of the casing main body 20 in a rotatable manner. Thecover 40 shown in FIGS. 1 and 2 includes a disc plate 41, a handle 42that protrudes upward from the disc plate 41, and a side wall 43integrally formed around the disc plate 41. The respective sections 41,42, 43 of the cover 40 are preferably made of a conductive resin and arepreferably integrally formed by injection molding. Of course, as withany of the e.emants described herein, other suitable materials andbethods of formation may be used. Eight engagement projections 45 areformed at an interval around the inner circumference of the side wall43. The engagement projections 45 engage with the outer ring 35 of theflange 33 (FIG. 1), so that the cover 40 is joined with the casing mainbody 20.

[0059] Referring to FIGS. 4 and 5, the torque mechanism 80 includes apair of main body ribs 32, 32 that protrude upward from the uppersurface of the outer tubular body 21 of the casing main body 20, acylindrical shaft 46, a pair of cover engagement projections 46 a, 46 aformed on the surface of the cylindrical shaft 46, a pair of covertrigger projections 47, 47 formed on the lower surface of the cover 40,a spring 82, and a torque plate 90. The cylindrical shaft 46 protrudesfrom the center of the lower surface of the cover 40. The pair of coverengagement projections 46 a, 46 a project from the outer circumferentialsurface of the cylindrical shaft 46. The pair of arc-shaped covertrigger projections 47, 47 protrude from the lower surface of the cover40. The pair of cover engagement projections 46 a, 46 a and the pair ofcover trigger projections 47, 47 are arranged symmetrically around therotational shaft of the cover 40.

[0060] The spring 82 is a coil spring interposed between the casing mainbody 20 and the torque plate 90. The spring 82 spans between the upperportion of the casing main body 20 and the outer circumferential portionof the torque plate 90 and accumulates the pressing force therein whenthe torque plate 90 is rotated counterclockwise relative to the casingmain body 20. Of course, other suitable biasing mechanisms could beemployed.

[0061] Referring to FIG. 5, the torque plate 90 is a thin disk platepreferably made of a resin, and has a through-hole and guide slots. Thetorque plate 90 has a center aperture 91 formed on the center thereof, apair of rib guides 93, 93 arranged concentrically with the centeraperture 91, and a pair of trigger guide slots 95, 95 arranged outsidethe rib guides 93, 93 and concentrically with the center aperture 91.The cylindrical shaft 46 of the cover 40 passes through the centeraperture 91. A pair of elastic torque pieces 94, 94 are formed aroundthe circumference of the center aperture 91. Each elastic torque piece94, 94 is a cantilever arc piece that extends from a support end 94 a,and has a plate engagement element 94 b that protrudes toward the centerof the aperture 91 and a slot 94 c formed on the outer circumferentialside of the plate engagement element 94 b. The elastic torque piece 94is elastically deformed to narrow the slot 94 c, when the plateengagement element 94 b is pressed against the cover engagementprojection 46 a of the cover 40 (see FIG. 7).

[0062] The pair of main body ribs 32, 32 fit in the pair of rib guides93, 93 arranged outside the pair of elastic torque pieces 94, 94. Eachof the main body ribs 32, 32 moves forwards and backwards along the ribguides 93, 93 between a first end 93 a and a second end 93 b of the ribguide 93. The pair of cover trigger projections 47, 47 fit in the pairof trigger guide slots 95, 95. Each of the cover trigger projections 47,47 moves forwards and backwards along the trigger guide slot 95 betweena first end 95 a and a second end 95 b of the trigger guide slot 95.

[0063] The following describes operation of the torque mechanism 80 inthe process of opening and closing the fuel supply inlet FNb of thefiller neck FN with the fuel cap 10. The torque mechanism 80 has pairsof elements symmetrically arranged around the rotational axis of thecover 40, although the following description generally refers to onlyone of each pair of elements for purposes of clarity of explanationonly.

[0064] The user may hold the handle 42 of the cover 40 (FIG. 2) with athumb and an index finger, position the casing engagement element 20 aformed on the casing main body 20 at the neck insertion notch FNd of thefiller neck FN, and insert the casing main body 20 in the axialdirection into the open fuel supply inlet FNb of the filler neck FN.When the handle 42 of the cover 40 is located in the vertical direction,the handle 42 of the cover 40 is designed to make the casing engagementelement 20 a readily fit in the neck insertion notch FNd and facilitatethe attachment of the fuel cap 10 to the filler neck FN. As shown inFIG. 6, in the torque mechanism 80, the pressing force of the spring 82causes the main body rib 32 to press against the first end 93 a of therib guide 93 and causes the cover engagement projection 46 a of thecover 40 to contact the plate engagement element 94 b of the torqueplate 90.

[0065] A clockwise rotational force is applied to the cover 40 in theabove state to close the fuel supply inlet FNb of the filler neck FNwith the fuel cap 10. The torque mechanism 80 carries out a series ofprocesses, as shown in FIGS. 6 through 9. The clockwise rotational forceapplied to the cover 40 is transmitted to the torque plate 90 throughthe engagement of the cover engagement projection 46 a of the cover 40with the plate engagement element 94 b of the torque plate 90 to rotatethe torque plate 90 clockwise. The clockwise rotation of the torqueplate 90 presses the main body rib 32 of the casing main body 20 againstthe first end 93 a of the rib guide 93 formed in the torque plate 90.This causes the cover 40, the torque plate 90, and the casing main body20 to rotate integrally in the closing direction of the fuel supplyinlet FNb. Accordingly, the force of engaging the casing engagementelement 20 a of the casing main body 20 with the opening engagementelement FNc of the filler neck FN is enhanced. When a reaction forcegenerated by the engagement force becomes equal to or greater than apredetermined rotational torque, the cover engagement projection 46 arides over the plate engagement element 94 b, as shown in FIG. 7, to afirst non-engaged state, shown in FIG. 8. This process to the firstnon-engaged state gives the user a feeling of attachment through anaudible click. The fuel supply inlet FNb of the filler neck FN isaccordingly closed with the fuel cap 10 under a predetermined clampingtorque.

[0066] In order to release the fuel cap 10 and open the fuel supplyinlet FNb of the filler neck FN, on the other hand, the user holds thehandle 42 of the cover 40 with the thumb and the index finger andapplies a counterclockwise rotational force to the cover 40, as shown inFIG. 11. The counterclockwise rotational force applied to the cover 40causes the cover engagement projection 46 a of the cover 40 to pressagainst the plate engagement element 94 b of the torque plate 90. Sincethe casing main body 20 is attached to the filler neck FN, only thecover 40 and the torque plate 90 are rotated counterclockwise againstthe pressing force of the spring 82. With the counterclockwise rotationof the cover 40 and the torque plate 90, the main body rib 32 movesalong the rib guide 93 towards the second end 93 b thereof.

[0067] The elastic torque piece 94 is easily deflected when the mainbody rib 32 is not positioned on the free end of the elastic torquepiece 94, as shown in FIG. 9. A further counterclockwise rotation of thecover 40, as shown in FIG. 10, causes the cover engagement projection 46a to press against the plate engagement element 94 b and elasticallydeform the elastic torque piece 94. The elastic deformation of theelastic torque piece 94 significantly deflects a part 94 d that contactsthe main body rib 32 and varies the width of the slot 94 c. This causesthe cover engagement projection 46 a to ride over the plate engagementelement 94 b with a smaller pressing force and results in a secondnon-engaged state, as shown in FIG. 11. Namely, the cover engagementprojection 46 a rides over the plate engagement element 94 b under asmaller rotational torque than the rotational torque required in theclosing process of the fuel cap 10 discussed above.

[0068] A counterclockwise rotation of the cover 40 to the position wherethe cover engagement projection 46 a rides over the plate engagementelement 94 b causes the cover trigger projection 47 to press against thefirst end 95 a of the torque plate 90. In this state, the main body rib32 presses against the second end 93 b of the rib guide 93. Therotational force applied to the cover 40 is accordingly transmitted tothe casing main body 20 via the cover trigger projection 47, the torqueplate 90, the second end 93 b of the rib guide 93, and the main body rib32. This causes the cover 40, the torque plate 90, and the casing mainbody 20 to rotate counterclockwise integrally.

[0069] The rotation of the casing main body 20 integrally with the cover40 by approximately 90 degrees (the state of FIG. 12) causes the casingengagement element 20 a to be released from the opening engagementelement FNc of the filler neck FN. The casing main body 20 isaccordingly released from the restraint force to the filler neck FN. Asthe casing main body 20 receives the pressing force of the spring 82 andthe cover 40 is held between the thumb and a finger of the user, thecasing main body 20 rotates counterclockwise relative to the cover 40and the torque plate 90 and returns to the initial position (the stateof FIG. 13). Here, the positional relationship between the handle 42 ofthe cover 40 and the casing engagement element 20 a of the casing mainbody 20 returns to the initial state.

[0070] The user has a feeling of attachment through an audible clickwhen the cover engagement projection 46 a rides over the plateengagement element 94 b in the process of closing the fuel cap 10. Thisassures the user that the fuel cap 10 is clamped under a predeterminedtorque. Namely, the fuel cap 10 is clamped under a fixed torque,irrespective of the elasticity of the gasket GS.

[0071] Since the engagement of the casing engagement element 20 a withthe opening engagement element FNc requires rotation of the fuel cap 10only approximately 90 degrees, a plurality of rotating actions by a useris not required to attach the fuel cap 10 to the filler neck FN.

[0072] In the torque mechanism 80 of the embodiment, when acounterclockwise rotational torque is applied to the cover 40 to openthe fuel cap 10, the plate engagement element 94 b and the coverengagement projection 46 a fall into the second non-engaged state, whichrequires a smaller rotational torque than the first non-engaged state.This does not interfere with the accumulation of the pressing force inthe spring 82. The accumulated pressing force enables the positionalrelationship between the cover 40 and the casing main body 20 to returnto the initial state.

[0073] Since this arrangement facilitates the positioning of the handle42 of the cover 40 and the casing engagement element 20 a of the casingmain body 20 in the process of closing the fuel cap 10, the fuel supplyinlet FNb of the filler neck FN is readily closed with the fuel cap 10.

[0074] The operation for bringing the cap engagement element 20 a intoengagement with the opening engagement element FNc during the closure ofthe fuel cap 10 will now be described with reference to FIGS. 14A-D.FIGS. 14A-D are diagrams depicting a series of operations for bringingthe cap engagement element 20 a into engagement with the openingengagement element FNc. When the fuel cap 10 is inserted into the fillerneck FN, the cap engagement element 20 a enters the neck insertion notchFNd. In other words, a condition is established in which the elementpasses through the state shown in FIG. 14A and penetrates deep into thenotch in the manner shown in FIG. 14B.

[0075] When the fuel cap 10 is subsequently rotated clockwise, the firstinclined portion 20 c is aligned with the opening engagement elementFNc, as shown in FIG. 14C (first engagement step). Further clockwiserotation of the fuel cap 10 causes the second inclined portion 20 d tomove in alignment with the opening engagement element FNc (as shown inFIG. 14D), and the fuel cap 10 to close the fuel supply inlet FNb of thefiller neck FN (second engagement step).

[0076] Assuming now that x is the distance traveled by the capengagement element 20 a in the direction of rotation and y is thedistance traveled in the axial direction, the corresponding tighteningrate TR can be defined as y/x. Assuming that the distances traveled inthe direction of rotation in the first and second engagement steps arex1 and x2, and the distances traveled in the axial direction are y1 andy2, the corresponding tightening rates can be defined as TR1 and TR2,respectively, as represented in FIGS. 14C and 14D. At this time, thesecond angle α2 of the first inclined portion 20 c is greater than thefirst angle α1 of the second inclined portion 20 d, so the tighteningrate TR1 of the first engagement step is greater than the tighteningrate TR2 of the second engagement step. Described below are the reasonsthat the tightening rate TR1 is set high in the initial first engagementstep, and the tightening rate TR2 is set low in the second engagementstep.

[0077]FIG. 15 is a graph showing the relation between gasket reactionand the amount of compression (tightening margin) during gasketflattening. The gasket reaction in FIG. 15 gradually increases duringinitial compression and then rises sharply during subsequentcompression. This indicates that a weak force is sufficient to achieveconsiderable gasket compression in the initial stage, but a strong forceis needed to sustain such compression after a certain level ofcompression has been achieved.

[0078] When the fuel cap 10 is closed by utilizing the gasket propertiesillustrated in FIG. 15, the gasket GS exerts a weak reaction force andthe torque mechanism 80 does not cause any slippage when the gasket GSis compressed at a high tightening rate TRI in the low-reaction firstengagement step. The reaction force of the gasket GS gradually increasesin the high-reaction second engagement step subsequent to the firstengagement step, but the torque mechanism 80 is still free from slippagebecause the gasket GS is gradually compressed at a low tightening rateTR2.

[0079] In the second engagement step, the cap engagement element 20 atravels over a considerable distance along the second inclined portion20 d in alignment with the opening engagement element FNc, creating anengagement state in which the opening engagement element FNc hasadequate play in the direction of rotation, and preventing the fuel cap10 from disengaging from the filler neck FN.

[0080] Accordingly, the steep first inclined portion 20 c is made toconform to the contours of the opening engagement element FNc during theinitial step (first engagement step) of compressing the gasket GS,whereby the gasket GS can be provided with an adequate tightening margin(and a tight seal can thus be obtained) without being subjected to astrong reaction force even when a narrow rotation angle is employed.

[0081] Another feature is that the cap engagement element 20 a movesfrom the right to the left in the drawings during the closure of thefuel cap 10, as shown in FIGS. 14B and 14C. Because the correspondingsecond angle α2 of the cap engagement element 20 a is greater than thefirst angle α1 of the opening engagement element FNc, the comer 20 e ofthe cap engagement element 20 a is not obstructed by the openingengagement element FNc when brought into contact with the openingengagement element FNc. The second inclined portion 20 d is made toconform to the contours of the opening engagement element FNc with asmall torque while remaining in a substantially uniform planar contacttherewith, as shown in FIG. 14D. The fuel cap 10 can thus rotatesmoothly without creating a large rotational torque between the firstinclined portion 20 c and the opening engagement element FNc.Accordingly, the fuel cap 10 can be closed with a wider rotation angle(that is, a wider tightening margin) when closed with the samerotational torque as in the prior art.

[0082] Yet another feature is that despite the fact that the capengagement element 20 a has an elaborate structure comprising a firstinclined portion 20 c and a second inclined portion 20 d, this structurecan still be easily formed with high precision by being injection-moldedmonolithically with the casing main body 20. A simpler formation processcan therefore be employed than when such an inclined surface is machinedinto a metal filler neck FN.

[0083] Thus, the inclinations α1 and α2 of the guide surface 20 b areappropriately set to optimum levels depending on the type of gasket orthe shape of the fuel cap. When, for example, the operating angle of thefuel cap is set to 60-120° and the tightening margin of the gasket GS to1.0-2.0 mm, the first angle α1 and the second angle α2 should be set to3-8° and 10-45°, respectively, and preferably to 3-6° (α1) and 10-30°(α2).

[0084] The first embodiment can be modified in the following manner.

[0085] First, although the above embodiment was described with referenceto a case in which a C-ring was used as a gasket, this option isnonlimiting, and an O-ring or other sealing member may also be used aslong as a weak reaction force is exerted during the initial compressionstage shown in FIG. 15.

[0086] Second, in addition to a rectilinearly inclined surface, a curvedinclined surface 20Bc such as the one shown in FIG. 16 can be providedto a casing engagement element 20Ba as a guide surface for the casingengagement element. In particular, adopting an approach suitable for thegasket characteristics shown in FIG. 15 can make the cap very easy tooperate without creating any sensation of sharp torque variations duringthe closure of the fuel cap.

[0087] Third, although the above embodiment was described with referenceto a case in which a gasket was mounted on a fuel cap, the gasket is notlimited in any particular way to this option and can be provided on theside of the filler neck as long such placement allows the movement ofthe fuel cap in the closing direction to create a seal for the gaparound the filler neck.

[0088]FIG. 17 shows the state existing before a fuel cap 110 (tank cap)pertaining to a second embodiment of the present invention is mountedover a filler neck FN, and FIG. 18 is a partial cross-sectional view ofthe fuel cap 110. In FIGS. 17 and 18, the fuel cap 110 is mounted over afuel supply inlet FNb (tank opening member) for supplying fuel to thefuel tank (not shown). The fuel cap comprises a casing main body 120preferably fashioned from polyacetal or another synthetic resinmaterial, a cover 40 preferably fashioned from nylon or anothersynthetic resin material and mounted in the upper portion of the casingmain body 120, a torque mechanism 80 (FIG. 18) interposed between thecover 40 and the casing main body 120, and a gasket GS (FIG. 18) mountedaround the outside of the upper portion of the casing main body 120 anddesigned to seal the gap around the filler neck FN.

[0089] The casing main body 120 houses a positive pressure valve and anegative pressure valve as relief valves (not shown). The gasket GS isprovided with a V-shaped slit GSs, is mounted in a gasket holder 121 inthe upper portion of the casing main body 120, and is designed to pressagainst a sealing surface FNf and to form a seal when the fuel cap 110is tightened inside the fuel supply inlet FNb.

[0090]FIG. 19 is a schematic diagram depicting the ribs formed on theouter circumferential surface of the casing main body 120. FIG. 19depicts the ribs and the like in a form in which the components arespread out in a plane extending substantially across a semicircle aroundthe casing main body 120. In FIG. 19, a cap engagement element 124 andmutually linked ribs are formed in the lower portion around the outsideof the casing main body 120. The ribs have a monolithic structurecomprising a lower guide rib 125, an upper guide rib 126, a pop-up rib127, a stopper rib 128, and a tilt-preventing rib 129. The ribs areprovided in pairs of the same size, with only one of each pair of ribsshown in FIG. 19. Of course, other configureations besides monolithicstructures in pairs can be employed by varying the formation and/ornumber of ribs.

[0091] The cap engagement element 124, shaped as a triangular projectionprovided with a guide surface 124 a on the top surface thereof,comprises a recess 124 b with a hollowed-out central portion, and adischarge slot 124 c provided below the recess 124 b. The discharge slot124 c is designed to prevent fuel from accumulating in the recess 124 b.The guide surface 124 a is inclined downward at an angle θ1 in theclockwise direction (in the direction from right to left in the drawing)and is configured in alignment with the lower surface of the openingengagement element FNc.

[0092] The lower guide rib 125 is disposed at an incline along the guidesurface 124 a and is designed to guide the opening engagement elementFNc. The upper guide rib 126 is disposed above the lower guide rib 125.A guide slot 130 for guiding the opening engagement element FNc isinterposed between the lower guide rib 125 and the upper guide rib 126.The lower guide rib 125 and upper guide rib 126 are designed to preventthe fuel cap 110 from penetrating too deep into the filler neck FN orretreating too far from the filler neck FN when the fuel cap 110 isturned too much and the opening engagement element FNc goes into theguide slot 130. The stopper rib 128, disposed in the right-side endportion of the guide slot 130, is designed to prevent excessive rotationof the opening engagement element FNc. The lower guide rib 125 can bedispensed with in locked structures in which the cap engagement element124 is in constant engagement with the opening engagement element FNcwhen the fuel cap 110 is mounted over the filler neck FN.

[0093] The pop-up rib 127 is formed as a continuation of the lower guiderib 125 and is inclined at a lesser angle θ2 than the guide surface 124a. Specifically, the components are configured such that the angle θ2 ofthe pop-up rib 127 is less than the angle θ1 of the guide surface 124 a,whereby this configuration allows the fuel cap 110 being rotated from aclosed state in the opening direction to be lifted by the openingengagement element FNc, and creates a force that moves the fuel cap 110upward (see FIG. 21B).

[0094] The tilt-preventing rib 129 is disposed underneath the gasket GS(FIG. 18) along the upper portion of the casing main body 120 and isdesigned to prevent the fuel cap 110 from being tilted by the outsideforce applied to the fuel cap 110, as will be described below. Theheight T2 of the tilt-preventing rib 129 is substantially equal to orgreater than the height T1 of the cap engagement element 124 in theradial direction.

[0095] The opening engagement element FNc is provided at two locationsin accordance with the position of the cap engagement element 124 in theinner circumferential portion of the filler neck FN, as shown in FIGS.17 and 19. A neck insertion notch FNd for allowing the cap engagementelement 124 of the fuel cap 110 to be inserted in the axial direction isdisposed in the gap formed by the opening engagement elements FNc. Whenthe fuel cap 110 is turned by a predetermined angle (about 90°) in theclockwise direction while the cap engagement element 124 is positionedin the neck insertion notch FNd and the fuel cap 110 is inserted intothe filler neck FN, the cap engagement element 124 moves along the lowersurface of the opening engagement element FNc and engages the openingengagement element FNc, causing the fuel cap 110 to be mounted over thefiller neck FN, as shown in FIG. 18.

[0096] Operations involved in the opening and closing of the fuel cap110 will now be described. The fuel cap 110 can be attached to anddetached from the filler neck FN by the torque mechanism 80 (FIG. 18) ata rotation angle of about 90°. The structure and operation of the torquemechanism 80 is described above.

[0097] Described first is the procedure for bringing the cap engagementelement 124 into engagement with the opening engagement element FNcduring the closure of the fuel cap 110. FIGS. 20A-D are diagramsdepicting a series of operations for bringing the cap engagement element124 into engagement with the opening engagement element FNc. When thefuel cap 110 is inserted into the filler neck FN, the cap engagementelement 124 enters the neck insertion notch FNd. In other words, a stateis reached in which the element passes through the state shown in FIG.20A and penetrates deep into the notch in the manner shown in FIG. 20B.

[0098] When the fuel cap 110 is subsequently rotated clockwise, theguide surface 124 a of the cap engagement element 124 moves in alignmentwith the lower surface of the opening engagement element FNc, as shownin FIG. 20C. When the fuel cap 110 is rotated through the closing anglein the manner shown in FIG. 20D, the fuel supply inlet FNb of the fillerneck FN is closed with the fuel cap 110, as shown in FIG. 18. At thistime, moving the fuel cap 110 inward will cause the gasket GS to becompressed by being pressed against the sealing surface FNf of thefiller neck FN. Excessive rotation of the fuel cap 110 is preventedbecause the fuel cap 110, even when rotated through a wide angle in theclosing direction, moves along the guide slot 130 and the stopper rib128 comes into contact with the opening engagement element FNc, as shownin FIG. 20D.

[0099] The operation for opening the fuel cap 110 will now be described.Rotating the fuel cap 110 counterclockwise from the state shown in FIG.21A causes the guide surface 124 a of the cap engagement element 124 tomove along the lower surface of the opening engagement element FNc. Inthe process, the guide surface 124 a is pressed against the lowersurface of the opening engagement element FNc by the righting force ofthe gasket GS. The compressed state of the gasket GS is released whenthe fuel cap 110 moves upward, with the cap engagement element 124gradually approaching the neck insertion notch FNd.

[0100] Before the cap engagement element 124 reaches the neck insertionnotch FNd, the cap engagement element 124 separates from the openingengagement element FNc, and the pop-up rib 127 slides over the uppersurface of the opening engagement element FNc, as shown in FIG. 21B.Specifically, the opening engagement element FNc moves from a state ofengagement with the cap engagement element 124 to a state of engagementwith the pop-up rib 127.

[0101] Moving the pop-up rib 127 counterclockwise along the uppersurface of the opening engagement element FNc causes the fuel cap 110move upward under the action of an upward force exerted by the openingengagement element FNc. The gasket GS shown in FIG. 18 is separated fromthe sealing surface FNf of the fuel supply inlet FNb by the upwardmovement of the fuel cap 110 during its rotation. Specifically, thegasket GS can be easily separated from the sealing surface FNf becauseof the concurrent action of an upward force during rotation relative tothe sealing surface FNf, even when considerable negative pressure existsinside the fuel tank. Accordingly, the gasket GS remains attached to thefuel cap 110 when the fuel cap 110 is removed.

[0102]FIG. 22 shows the state in which an outside force is applied tothe fuel cap 110 from below, and FIG. 23 shows the effect demonstratedby the outside force shown in FIG. 22 when this force is applied to thefuel cap 110, with the fuel cap 110 being viewed from above. In the caseof the fuel cap 110, the upward outside force fa shown in FIG. 23 issometimes accidentally applied to the cover 40. In such cases, only asmall moment is exerted relative to the cap engagement element 124, andthe fuel cap 110 is prevented from tilting at a steep angle because theoutside force fa is applied in the vicinity of the position Pa ofengagement between the cap engagement element 124 and the openingengagement element FNc.

[0103] By contrast, applying the upward outside force fb shown in FIG.23 will cause a substantial moment to be exerted relative to the capengagement element 124, and a large tilting force to be applied to thefuel cap 110 because the outside force fb is applied at a distance fromthe position Pa of engagement between the cap engagement element 124 andthe opening engagement element FNc. However, as seen in FIG. 18, theheight T2 of the tilt-preventing rib 129 is greater than the height T1of the cap engagement element 124, so the rib comes into contact withthe inner wall of the filler neck FN before the cap engagement element124 comes into contact with the inner wall of the filler neck FN,limiting the tilting of the fuel cap 110.

[0104] When the casing main body 120 is shaped as a cylinder, and thecap engagement element 124 is shaped as a projection in this manner, agap is left between the outer circumferential surface of the casing mainbody 120 and the inner wall of the filler neck FN in the area notcovered by the cap engagement element 124, but filling the gap with thetilt-preventing rib 129 prevents the fuel cap 110 from chattering underthe action of an outside force.

[0105] In addition, the ribs can be easily formed with high precision bybeing injection-molded monolithically with the casing main body 120. Asimpler formation process can therefore be employed than when such aninclined surface is machined into a metal filler neck FN.

[0106] The present invention is not restricted to the above embodiment,but there may be many modifications, changes, and alterations withoutdeparting from the scope or spirit of the main characteristics of thepresent invention. Some examples of possible modification are givenbelow.

[0107] The embodiments discussed above relate to the tank cap for thefuel tank of the automobile. The principle of the present invention is,however, applicable to any tanks of any structures, as long as the tankopening member is closed with a cap.

What is claimed is:
 1. A tank cap apparatus, comprising: a tank openingmember having an opening engagement element and a sealing surface; a capconstructed and arranged to close the tank opening member throughrotation about a rotation axis, the cap including a cap engagementelement for engaging the opening engagement element; and a gasketinterposed between the cap and the sealing surface of the tank openingmember to seal a gap therebetween; wherein the opening engagementelement is inclined at a predetermined angle relative to a directionorthogonal to the rotation axis of the cap, the predetermined anglebeing an angle at which the gasket is compressed according to a closingoperation of the cap; and wherein the cap engagement element has a guidesurface that engages with the opening engagement element by a rotationof the cap in a closing direction while the cap is inserted into thetank opening member, an angle of the guide surface for contacting withthe opening engagement element being constructed and arranged todecrease with a rotation of the cap during the closing operation.
 2. Thetank cap apparatus in accordance with claim 1, wherein the guide surfacehas a first inclined surface and a second inclined surface inclinedrespectively at an angle of a direction orthogonal to the rotation axisof the cap, an angle of the first inclined surface being greater thanthe predetermined angle of the opening engagement element, an angle ofthe second inclined surface being substantially equal to thepredetermined angle.
 3. The tank cap apparatus in accordance with claim2, wherein the gasket is a C-ring.
 4. The tank cap apparatus inaccordance with claim 1, wherein the tank cap apparatus is configuredsuch that the cap engagement element is engaged with the openingengagement element by rotation of the cap by an angle of 180° or less.5. The tank cap apparatus in accordance with claim 4, further comprisinga tilt-preventing rib formed on an outer circumferential portion of thecap between the cap engagement element and the gasket, thetilt-preventing rib being constructed and arranged to come into contactwith an inner wall of the tank opening member, thereby preventing thecap from tilting upon application of a cap-tilting force.
 6. The tankcap apparatus in accordance with claim 5, wherein the tilt-preventingrib is disposed above an outer circumferential portion of the capexclusive of the cap engagement element.
 7. The tank cap apparatus inaccordance with claim 6, wherein the tilt-preventing rib extends to aradial height substantially equal to or greater than a radial height ofthe cap engagement element.
 8. A tank cap apparatus, comprising: a tankopening member having an opening engagement element and a sealingsurface; a cap constructed and arranged to close the tank opening memberthrough rotation about a rotation axis, the cap including a capengagement element for engaging the opening engagement element; and agasket interposed between the cap and the sealing surface of the tankopening member to seal a gap therebetween; wherein the openingengagement element is inclined at a predetermined angle relative to adirection orthogonal to the rotation axis of the cap, the predeterminedangle being an angle at which the gasket is compressed according to aclosing operation of the cap; and wherein the cap engagement element hasa guide surface that engages with the opening engagement element by arotation of the cap in a closing direction while the cap is insertedinto the tank opening member, an angle of the guide surface forcontacting with the opening engagement element being constructed andarranged to be greater than the predetermined angle of the openingengagement element.
 9. The tank cap apparatus in accordance with claim8, wherein the gasket is a C-ring.
 10. The tank cap apparatus inaccordance with claim 8, wherein the tank cap apparatus is configuredsuch that the cap engagement element is engaged with the openingengagement element by rotation of the cap by an angle of 180° or less.11. The tank cap apparatus in accordance with claim 10, furthercomprising a tilt-preventing rib formed on an outer circumferentialportion of the cap between the cap engagement element and the gasket,the tilt-preventing rib being constructed and arranged to come intocontact with an inner wall of the tank opening member, therebypreventing the cap from tilting upon application of a cap-tilting force.12. The tank cap apparatus in accordance with claim 11, wherein thetilt-preventing rib is disposed above an outer circumferential portionof the cap exclusive of the cap engagement element.
 13. The tank capapparatus in accordance with claim 12, wherein the tilt-preventing ribextends to a radial height substantially equal to or greater than aradial height of the cap engagement element.
 14. A tank cap for closinga tank opening member provided with an opening engagement element formedin an inclined state at a predetermined angle, comprising: a casing mainbody including (i) a flange formed in an upper portion the casing mainbody, (ii) a gasket holder disposed underneath the flange, and (iii) acap engagement element formed underneath the gasket holder; a gasketheld by the gasket holder, the gasket sealing a gap around a sealingsurface of the tank opening member; and a cover rotatably mounted overthe flange; wherein the cap engagement element has a guide surfaceconstructed and arranged to be engaged with the opening engagementelement by a rotation of the cap in a closing direction while the tankcap is inserted into the tank opening member, an angle of the guidesurface for contacting with the opening engagement element beingconstructed and arranged to decrease with a rotation of the cap duringthe closing operation.
 15. The tank cap in accordance with claim 14,wherein the guide surface has a first inclined surface and a secondinclined surface inclined respectively at an angle of a directionorthogonal to the rotation axis of the cap, an angle of the firstinclined surface being greater than the predetermined angle of theopening engagement element, and an angle of the second inclined surfacebeing substantially equal to the predetermined angle.
 16. The tank capin accordance with claim 14, wherein the tank cap is configured suchthat the cap engagement element is engaged with the opening engagementelement by rotation of the cap by an angle of 180° or less.
 17. The tankcap in accordance with claim 16, further comprising a tilt-preventingrib formed on an outer circumferential portion of the casing main bodybetween the cap engagement element and the gasket, the tilt-preventingrib being constructed and arranged to come into contact with an innerwall of the tank opening member, thereby preventing the tank cap fromtilting upon application of a cap-tilting force.
 18. The tank cap inaccordance with claim 17, wherein the tilt-preventing rib is disposedabove an outer circumferential portion of the casing main body exclusiveof the cap engagement element.
 19. A tank cap for closing a tank openingmember provided with an opening engagement element formed in an inclinedstate at a predetermined angle, comprising: a casing main body including(i) a flange formed in an upper portion the casing main body, (ii) agasket holder disposed underneath the flange, and (iii) a cap engagementelement formed underneath the gasket holder; a gasket held by the gasketholder, the gasket sealing a gap around a sealing surface of the tankopening member; and a cover rotatably mounted over the flange; whereinthe cap engagement element has a guide surface constructed and arrangedto be engaged with the opening engagement element by a rotation of thecap in a closing direction while the tank cap is inserted into the tankopening member, an angle of the guide surface for contacting with theopening engagement element being constructed and arranged to be greaterthan the predetermined angle of the opening engagement element.
 20. Thetank cap apparatus in accordance with claim 19, wherein the gasket is aC-ring.
 21. The tank cap apparatus in accordance with claim 19, whereinthe tank cap apparatus is configured such that the cap engagementelement is engaged with the opening engagement element by rotation ofthe tank cap by an angle of 180° or less.
 22. The tank cap apparatus inaccordance with claim 21, further comprising a tilt-preventing ribformed on an outer circumferential portion of the casing main bodybetween the cap engagement element and the gasket, the tilt-preventingrib being constructed and arranged to come into contact with an innerwall of the tank opening member, thereby preventing the tank cap fromtilting upon application of a cap-tilting force.
 23. The tank capapparatus in accordance with claim 22, wherein the tilt-preventing ribis disposed above an outer circumferential portion of the casing mainbody exclusive of the cap engagement element.
 24. The tank cap apparatusin accordance with claim 23, wherein the tilt-preventing rib extends toa radial height substantially equal to or greater than a radial heightof the cap engagement element.